There are many web tools that are developed to help web developers in the web development. These web tools are commonly called add-ons or extensions. Firebug is definitely one of the most popular in-browser tools usually used by many web developers especially for Firefox users. Firebug is a Firefox extension that allows you to inspect and diagnose HTML, CSS, Ajax and JavaScript codes.
With Firebug, you can view the results of HTTP connections and visualize your Cascading Style Sheet or CSS to formulate the website layout.

Firebug interface

Firebug interface

Firebug is an open source application and comes with a JavaScript console, logging API, and a useful Network Monitor necessary for HTTP connections.

Firebug is also a very helpful tool for AJAX application developers because it is capable of instant edits on the Document Object Model (DOM) to see what happens when you manipulate Web page elements after a user action.

Other common in-browser tools are Web Developer, YSlow and Fiddler Web Debugger.